The abnormal cell surfaces of embryonic and spermatogenic cells from animals carrying lethal mutations at the T/t-complex in the mouse, and their normal counterparts, are studied by methods of serology and light and electron microscopy. By cytotoxicity tests, we have detected t-antigens on cells from 8-day mutant embryos. Studies using immunoperoxidase techniques on sections are under way to reveal tissue distribution of the antigens. Using anti-tw18 serum and antibody coated latex spheres, a proportion of primitive streak embryos from segregating litters show labeling in the scanning electron microscope and, significantly, the label appears only over ectoderm and primitive streak, the tissues that are specifically affected in the mutant embryo. Scanning and transmission EM views show that the cell surface is abnormal in tw32/tw32 morulae; microvilli of neighboring blastomeres do not become closely intertwined as in normal morulae that are undergoing compaction. It has also been observed by SEM and TEM that extracellular matrix components around the neural tube of 9-day T/T embryos are abnormally clumped together. Abnormalities in the redistribution of receptors for concanavalin A have been observed by fluorescence microscopy on spermatozoa from sterile mice carrying two different lethal t haplotypes during sperm maturation in the epididymis and in vitro.